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bwilson4web
02-18-2012, 07:41 PM
Hi,

I am rebuilding a 1987 built, Viking Dragonfly, a two-place, foam-and-fiberglass plane that came with a 60 hp, 1983 era, 1835cc VW engine. In 25 years, there have a few technical improvements so I did a 'trade study' to find engine options that (1) improve useful load, and (2) preserve the 500 mile range. But when I announced the results, some of the reactions were not technical.

My trade study came up with three candidates:

625 cc Hirth 3503 - two cycle, water cooled, oil injected, fuel injection, geared, two cylinders.
1800 cc Jibaru - four cycle, air cooled, carb, four cylinders.
680 cc HKS 700E - four cycle, air cooled, carb, geared, two cylinders.
Details about my project, requirements and study are here:
http://hiwaay.net/~bzwilson/dragonfly/

I need to find productive sources about two cycle engines and had already joined the YahooGroup Hirth group. But I suspect there may be other forums and sources with hands-on, experience with larger, two cycle engines. Any recommendations?

You know the funny thing is I can be persuaded by facts and data but I can't be brow-beaten.

Thanks,
Bob Wilson

Dana
02-19-2012, 08:26 AM
There's a 2-stroke section on homebuiltairplanes.com (http://homebuiltairplanes.com).

2-strokes have their advantages and disadvantages. Advantages are better power to weight ration than any 4-stroke, simpler construction, and lower cost. Disadvantages are higher fuel consumption (so maintaining range compared to a 4-stroke might be a problem), need to mix oil and gas, and and critical mixture (run too lean and the engine fries, too rich and you foul a plug). Also many (not all) 2-strokes are single ignition, which reflects the snowmobile heritage of many designs.

A lot (but not all) of 2-strokes bad reputation is from the early days when most 2-strokes were straight snowmobile conversions. People tried to ignore them and treat them like a Contentinal (add fuel, turn the key, and push the throttle forward), and had troubles. They do require more attention than a 4-stroke, with seasonal carburetor jetting adjustments and preventive maintenance. As one example, a leaking crankcase shaft seal: in a 4-stroke, you lose a bit of oil and maybe have to wipe it off, while on a 2-stroke it's an air leak that fries your engine.

But even though I've been flying 2-strokes for over 10 years (and never had an engine failure related to the fact that it's a 2-stroke), I'd install a 4-stroke if feasible.

turtle
02-19-2012, 01:39 PM
There's a 2-stroke section on homebuiltairplanes.com (http://homebuiltairplanes.com).

2-strokes have their advantages and disadvantages. Advantages are better power to weight ration than any 4-stroke, simpler construction, and lower cost. Disadvantages are higher fuel consumption (so maintaining range compared to a 4-stroke might be a problem), need to mix oil and gas, and and critical mixture (run too lean and the engine fries, too rich and you foul a plug). Also many (not all) 2-strokes are single ignition, which reflects the snowmobile heritage of many designs.

A lot (but not all) of 2-strokes bad reputation is from the early days when most 2-strokes were straight snowmobile conversions. People tried to ignore them and treat them like a Contentinal (add fuel, turn the key, and push the throttle forward), and had troubles. They do require more attention than a 4-stroke, with seasonal carburetor jetting adjustments and preventive maintenance. As one example, a leaking crankcase shaft seal: in a 4-stroke, you lose a bit of oil and maybe have to wipe it off, while on a 2-stroke it's an air leak that fries your engine.

But even though I've been flying 2-strokes for over 10 years (and never had an engine failure related to the fact that it's a 2-stroke), I'd install a 4-stroke if feasible.
Well said.

The only two-strokes with a prop I work on are airboats. Lots of fried engines every summer. All due to lack of maintenance or improper operation. That said, because they are not very forgiving, I'd prefer not to fly behind one. Flying requires enough concentration and attention to detail as it is. No need to make it harder if an alternative exists. Most of my engines come from crashed ultralights where a piston melted.

I have to question the requirement to maintain the 500 mile range, even if it means switching to a less reliable engine. From the website: "Adding the ballistic chute means 'reliability' gets a reduced valuation when the final engine selection is made." I'm shocked to hear someone who is analyzing this to death and will only accept hard data, jump to this conclusion. Deploying the chute is not free. There is no guarantee it will work successfully. If it does there will be airframe damage, the cost to repack the chute and recovery of the aircraft. There may be injuries or death or third party damage depending on where it comes down. If you want to crunch some numbers, figure how often you will be using the 500 mile range vs. shorter flights. Is staying in the air for an extra, say, 1/2 hour, worth the added risk and tremendous cost for the few times it will be used? If so, is the Dragonfly really the right plane for your mission or are you trying to beat it into submission?

bwilson4web
02-19-2012, 05:57 PM
Thank you!

I had visited them briefly when Google pointed to a couple of postings but moved on. I went back and sure enough I'm finding serious two-cycle information. I like the EAA and certainly it has a wealth of information and expertise but I'm finding it is also a good source of pointers to other technical resources.

Bob Wilson

bwilson4web
02-19-2012, 06:44 PM
. . .
I have to question the requirement to maintain the 500 mile range, even if it means switching to a less reliable engine.
I bought this plane to move a maximum, useful payload at least 500 miles in a 'bladder's time.' N19WT has over 100 hours of acceptable performance that I plan to maximize using current technology.

I've added emphasis to this quoted material to make sure everyone understands what we agree about:

. . .
From the website: "Adding the ballistic chute means 'reliability' gets a reduced valuation when the final engine selection is made." I'm shocked to hear someone who is analyzing this to death and will only accept hard data, jump to this conclusion. Deploying the chute is not free. There is no guarantee it will work successfully. If it does there will be airframe damage, the cost to repack the chute and recovery of the aircraft.
Yes, we fully agree. We agree about the technical challenges of adding a ballistics parachute. The alternative is to have parachutes used as the seat cushions for both me and the passenger and "GOOD LUCK!" to everyone on the ground.

This part is special:

. . .
There may be injuries or death or third party damage depending on where it comes down.
So the options are:

four-stroke engines never fail
airplanes gliding down at 60+ mph never crash and injure those below
flying at night and into IFR weather never leads to accidents
This is an especially foolish claim when considering the alternatives. I have no problem with a mandatory, insurance driven requirement for a tested, ballistics parachute. In the meanwhile, I choose safety.


. . .
If you want to crunch some numbers, figure how often you will be using the 500 mile range vs. shorter flights. Is staying in the air for an extra, say, 1/2 hour, worth the added risk and tremendous cost for the few times it will be used?
Yes, the cost is well within my budget and the requirements can be met by:

Hirth 3503 - best combination of payload and range
HKS 700e - better payload and less but acceptable range
Jabiru - acceptable payload and range
Yes, the Dragonfly really is the right plane for my mission. I have long wondered about modern two-cycle engines and admired the safety aspects of ballistics parachutes. Both are commonly used in ultralight aircraft. BTW, this is the Ultralight discussion group where such technologies are common. Will you also disrespect the other ultralights too?


. . . If so, is the Dragonfly really the right plane for your mission or are you trying to beat it into submission?
Since I bought N19WT, I had not considered 'beating' to be effective aircraft or engineering approach. But I am Southern which means I can be persuaded but I won't be brow-beaten into your point of view.

Bob Wilson

turtle
02-19-2012, 08:55 PM
Nice job at twisting words. On your website you imply that a ballistic chute can compensate for a less reliable engine. "Adding the ballistic chute means 'reliability' gets a reduced valuation when the final engine selection is made." So you are fine with pulling the chute and causing whatever carnage you may cause just so you can get a little more distance. This isn't about installing a chute as a secondary, incidental safety measure. The reasoning on your site is to allow you to create an unsafe condition and be able to recover from it.

There is no disrespecting ultralights here. They serve a purpose but they do have a higher risk. I don't think anyone will deny that. Even the manufacturers of the engines put a low TBO on them because of their failure history. What people don't consider is that we are all in this together. An accident due to someone cutting corners or hotdogging affects everyone from the UL flyer to the millionaire with the Turbo Beaver. It's up to us to choose safety over function. If someone needs to reduce tubing thickness and thereby reduce the safety factor to get more useful load, he chose the wrong aircraft. If the range and load aren't enough without sacrificing reliability, he chose the wrong aircraft. Spec sheets and computer modeling don't keep you in the air. Ask Airbus with the 380. The high-tech Rotax 914 is a poor substitute for the ancient IO-240, as was proven by flight schools across the country with the DA20. Throwing technology at a problem that was caused by poor planning is a bad idea. Using armchair engineering to justify reducing the safety level is worse.

Useful load - Range - Reliability

You can only pick two.

bwilson4web
02-19-2012, 11:33 PM
Nice job at twisting words. . . .
No, thank you for providing such easy words. Still I understand you may have other questions about my project but this is the wrong thread.

If you would like to go through a semi-formal, NASA style review, let me know. I'll send a note to the moderators and ask which forum they would prefer to see the reviews posted.

Thanks,
Bob Wilson

bwilson4web
02-20-2012, 01:05 AM
There's a 2-stroke section on homebuiltairplanes.com (http://homebuiltairplanes.com).
. . .
As follow-up, I got a recommendation about Ultralight Propulsion by Glenn Brinks, 1982, and ordered a copy. I have some ideas to test and this may provide some additional technical details.

Thanks again,
Bob Wilson

Dana
02-20-2012, 07:50 AM
I too question the relationship between a BRS and engine reliability. A BRS is not a substitute for a reliable engine. A BRS should not even be considered an option in case of engine failure... under nearly all circumstances, it's better to fly the aircraft to a forced landing than to pull the chute and become a passenger in what will certainly be a hard touchdown. You use the chute when the wings fall off, or you have a midair collision, not when the engine quits (unless, perhaps, you're IFR over ground fog or on a dark night, in which case the choice of anything but the most reliable engine might be questioned).

bwilson4web
02-20-2012, 09:15 AM
I too question . . .
That is OK but I came here to discuss two-cycle engines. If you are interested in a NASA style, project review of N19WT, let me know. I've already sent a note to the moderators asking where I should post it.

Bob Wilson

Racegunz
02-20-2012, 10:43 AM
Hey Bob, I just wanted you to know I for one am interested in the experimental side of EAA (unlike some others who need to stick with certified rigs)and have flown behing a 2 stroke, I was a skeptic but after selling it and aquiring a "reliable" continental powered aircraft, I have realized that all that reliability hinges on the owner. The Rotax I flew with is a reliable design and after I purged the previous owners influence on it it was a fantastic engine, but I was swayed by the crowd to get a "real" airplane and engine. The Continental failed at 75 hrs, less than 200 since major once again the previous owner's influence not the design. With my practical experience having flown behind one and having done all the set-up and maintenance, I say the 2 strokes are a fine choice for a lightweight powerplant. As long as the EGT's are monitored during tuning and proper maintenance done there is no secret to the 2 strokes operation if you run it lean it will melt.

steveinindy
02-20-2012, 11:20 AM
"GOOD LUCK!" to everyone on the ground.

The risk to folks on the ground is so low as to be negligible. One researcher put the odds of a given person being killed by a crashing aircraft while not involved in aviation related activities at roughly the same likelihood as being struck by lightning while holding the day's winning lottery ticket.

rampil
02-21-2012, 06:41 PM
Just a quick point of clarification:
The DA-20 was powered by the normally aspirated 80 hp 912, not the more high tech 115 hp turbo 914 engine.

Where the A&Ps were trained to understand the water cooling and the different metallurgy, the engines and schools
ran fine. I can cite Diamond Aviation at KSQL in the 1990s as an example. Not so much high tech as just different tech.

From what I saw and heard along the way, treating a Rotax like a Lycosaurus respects neither and is a sure path
to trouble. This seemed to happen in lots of places where factory training was not taken.

How often do you hear of a 4 stroke Rotax throwing a rod? Or a Lyco floating and crushing its valves at 6000 rpm?

Back to 2Strokes, I have never used them in aircraft, only in yard tools.
Engineering is always compromise, in this case reliability exchanged for weight and performance.
I have ocassionally had my 912s take me on 2000 mile round trips, some thing I would not contemplate with a
two stroke.

Racegunz
02-22-2012, 08:03 AM
Back to 2Strokes, I have never used them in aircraft, only in yard tools.
Engineering is always compromise, in this case reliability exchanged for weight and performance.
I have ocassionally had my 912s take me on 2000 mile round trips, some thing I would not contemplate with a
two stroke.

Well without having flown behind one, kinda quick to not contemplate, you pointed out others that probably haven't owned a 912/914 pointed out problems with them,pot.....kettle???

radfordc
02-23-2012, 07:51 PM
There is no disrespecting ultralights here. They serve a purpose but they do have a higher risk. I don't think anyone will deny that.

I personally felt safer in my ultralights than I do flying my Sonex. Yes, I had 10 "emergency" landings in ultralights over the years, but because they land so slow and in such a short distance and because you only fly ULs over "safe" terrain there was never any fear of damage or injury. Basically, the engine quits and you land.

steveinindy
02-23-2012, 07:54 PM
Likewise. I'd take an engine out in an UL aircraft any day over a Sonex.

bwilson4web
02-24-2012, 05:40 AM
I finally got around to weighing the 60 hp, 1835cc, HAPI VW engine: 172.6 lbs. This is about 20 lbs heavier than the 'dry weight' in the Sport Aviation article. This weight includes the starter, cooling baffles, intake manifold, magneto, backup electric ignition, and second spark plugs. It did not include the 1.3 lb Ellision and 7.0 lb exhaust stacks for an all-up weight of 179.9 lbs. BTW, I will be selling the HAPI but not the Ellison, yet.

The BSR straps can run from the engine mount points along the fuselage and back to the wing cover. Replacing the VW engine means the 'side cheeks' are no longer needed or could be replaced by something more functional:
http://hiwaay.net/~bzwilson/dragonfly/2012/firewall_010.jpg
For example, ducts that carry radiator plumbing and BSR stainless steel cable.

One radical thought is the BSR might fit in front of the firewall. This improves the CG by moving it forward and putting the rocket on the other side of the firewall. In fact, a canister BSR might fit:
http://hiwaay.net/~bzwilson/dragonfly/2012/firewall_020.jpg
A lighter, 60 hp engine means the engine mount needs to extend forward to maintain CG. This increases the space between the firewall and engine and possibly allowing a BSR canister to fit. This space might compete with a radiator and ducting but there is a more radical solution.

The coolant hoses could route outside the sides of the fuselage back to a rear mounted radiator and ducting in the fuselage behind the wing drag bulkhead. This avoids running plumbing through the cabin and bulkheads. Jon Finley did this with his Subaru powered, Q2 running the pipes under the fuselage.

The radiator hoses could be encased in functional, external duct(s) on each side with a profile no worse than the 3" x 9" side cheeks. The side cheeks could be the front anchor for aluminum ducting. Run the BSR straps or stainless steel cable along the duct top and two problems are solved. A rear, P-51 style, radiator offers the possibility of reduced cooling drag. Both options need to be modeled.

Bob Wilson

ehawkins
02-24-2012, 10:11 AM
Bob:

2-cycle engines of today are much better than in previous years due to being designed and built for aviation use specifically. The improvements include automatic oil injection to eliminate mixing errors with fuel, dual electronic ignition, and gear-driven PSRU"s to slow the prop speed down and eliminate drive belt failures.

I have flown 2-cycle engines in ultralights and homebuilts since 1980 and I have personally experienced engine failure while airborne. My experience is definitely skewed due to the fact that many of these failures
were during test flights of newly completed builds.

It is true that the 2-cycle requires more monitoring by the owner, but the advantages in power to weight ratio is often worth the trade-off. The key to safety is to educate yourself on the characteristics of the 2-stroke engine and how to avoid the pitfalls that I see so many 4-stroke users fall into due to ignorance or complacency.

My personal Hi-Max has a Rotax engine and, with proper care and monitoring has performed flawlessly for over 200 hrs to date. This engine was purchased used and had points ignition. Before flying this a/c, I removed the engine from the plane & checked out the compression, crankshaft seals and points. I replaced all bearings, points, and belts before reinstalling the engine for run testing. After adjusting the carb to the proper egt & cht temp ranges, the engine has been perfect. Just be aware that ambient temperature and humidity changes will require carb adjustments to maintin proper mixture settings. None of these adjustments are difficult and take only several minutes to make.

bwilson4web
02-24-2012, 10:23 AM
Thanks Ed,
2-cycle engines of today are much better than in previous years due to being designed and built for aviation use specifically. The improvements include automatic oil injection to eliminate mixing errors with fuel, dual electronic ignition, and gear-driven PSRU"s to slow the prop speed down and eliminate drive belt failures.. . . I'm also going with factory installed fuel injection to manage the mixture. But I have no illusions that any engine is 100% reliable plus I remember how I flew my ~320 hours.
Bob Wilson

bwilson4web
02-25-2012, 08:16 AM
Good luck to you Bob.
Thanks Jeff,

I also noticed my Yahoo Dragonfly list membership was pulled. Fortunately N19WT is not dependent upon the Dragonfly list.

Too bad, I was going to share progress on my airframe and navigation light work.

Bob Wilson

martymayes
02-25-2012, 09:51 AM
That is OK but I came here to discuss two-cycle engines. If you are interested in a NASA style, project review of N19WT, let me know. I've already sent a note to the moderators asking where I should post it.

Bob Wilson

I'd definitely be interested in seeing such a review Bob. In particular, I'd like to see what kind of risk is incurred by flying at night in a homebuilt airplane with a converted auto powerplant or two-stoke engine.

Enemy Ace
02-28-2012, 03:30 PM
An EAA Webinar recently discussed two-stroke failure rates. It seems that the lowest rate of engine failure was in slow, draggy aircraft such as powered parachutes. Next highest was ultralights, followed by slow light aircraft such as trikes and Kitfoxes. Highest of all were clean, fast airframes. Seems that two-strokes can have a kind of lean-mixture runaway when the prop unloads, which is more likely in a fast plane (like a Dragonfly). Here's a link to the Webinar on Youtube: http://www.youtube.com/watch?v=GaaLb-BzqT4
I'd like to put in a plug for HKS: I've spoken with pilots who fly behind HKS engines and love 'em. Many of them flew two-strokes previously.

Wayne

bwilson4web
02-29-2012, 02:28 AM
Thanks!

An EAA Webinar recently discussed two-stroke failure rates. It seems that the lowest rate of engine failure was in slow, draggy aircraft such as powered parachutes. Next highest was ultralights, followed by slow light aircraft such as trikes and Kitfoxes. Highest of all were clean, fast airframes. Seems that two-strokes can have a kind of lean-mixture runaway when the prop unloads, which is more likely in a fast plane (like a Dragonfly). Here's a link to the Webinar on Youtube: http://www.youtube.com/watch?v=GaaLb-BzqT4
I'd like to put in a plug for HKS: I've spoken with pilots who fly behind HKS engines and love 'em. Many of them flew two-strokes previously. . . .
This is the type of facts and data that help everyone move forward. It is only by looking at two-cycle failures that we can see if there are ways to mitigate the risks. I'll take a peek at the Webinar and see if I can contact the author(s) with my mitigation plan. I'll share what I learn here, later.

The HKS is still in the running because it just meets the range requirement. Unfortunately it is rated at 60 hp for only for three minutes. That is enough for take off and to gain a little altitude so it is still in the running if the 3502 preliminary design hits a show stopper.

My engineering practices require a fairly detailed design, a preliminary design review (PDR) that defines everything necessary for an engineering solution. It includes the parts list and prices. This Preliminary Design is then presented and if no 'show stoppers' show up, the parts are ordered.

BTW, I'm also working on what is called a Mission review but I'm not yet ready to publish in part due to being pretty busy an translating from NASA system engineering terms to 'human'. But it is a good exercise because it sets of goals that the PDR needs to cover the requirements.

Thanks!
Bob Wilson

ehawkins
02-29-2012, 09:12 AM
Thanks Ed, I'm also going with factory installed fuel injection to manage the mixture. But I have no illusions that any engine is 100% reliable plus I remember how I flew my ~320 hours.
Bob Wilson



Folks who think *any* engine is 100% reliable need to be grounded for ignorance. Due diligence MUST be exercised in maintaining and operating all systems of the a/c especially the engine if safe flight is to be accomplished. Education and training are the key. There is no excuse, in my opinion, for any pilot to attempt to fly an a/c without proper familiarization with the a/c and all of its operating characteristics.

Personal prejudice as to equipment choices are common, but education and familiarization with the available engine choices out there open up design possibilities not available to one-engine type mentalities. This is one of the reasons that the experimental market is the fastest growing segment of private aviation in the country today.

Dana
02-29-2012, 01:20 PM
FWIW, and anecdotal evidence such as this means nothing, but I've had two complete engine failures with a certified Continental engine (one mechanical failure, one contaminated fuel, both resulting in forced landings) and two with my 2-stroke Cuyuna (both of which I believe to be carburetor icing, and in both cases I got it cleared up before having to land, though once it was on short final to a horse farm pasture) in roughly the same number of flight hours.

Certainly the 2-stroke takes more attention to maintenance to achieve that level of reliability, and the TBO is shorter, but it hasn't been onerous.

bwilson4web
03-03-2012, 11:10 AM
Thanks Wayne,


. . . Seems that two-strokes can have a kind of lean-mixture runaway when the prop unloads, which is more likely in a fast plane (like a Dragonfly). Here's a link to the Webinar on Youtube: http://www.youtube.com/watch?v=GaaLb-BzqT4
. . .
I will contact them to learn more but I'm also well aware that motocross racing is (or has been) dominated by motorcycles with powerful, 2-stroke engines. Talk about 'off loading,' those folks regularly fly off the tops of bumps or humps and I find it difficult to believe that 'unloading' automatically leads to a lot of motocross engine failures. I'm not disputing the Webinar's observations as much as sharing that other 2-stroke engines seem to deal with 'off loading' without similar failures. It is a puzzle that needs understanding and insights.

Thanks again for the link. I have another data point.

Bob Wilson

Dana
03-03-2012, 08:59 PM
There's a difference between a bike or kart unloading momentarily as it flies over a bump and an aircraft on a long unloaded descent...

bwilson4web
03-03-2012, 11:46 PM
There's a difference between a bike or kart unloading momentarily as it flies over a bump and an aircraft on a long unloaded descent... Agreed which suggests the failure is not quite as rapid as the Webinar suggested when it discussed transition to cruise leading to prop unloading. Also, I don't know if two-stroke, motorcycles are able to go down large hills and mountains. I have a friend who races them and I'll ask.

This is why I need to ask the webinar speaker a follow-up question. One of the reasons I'm looking at oil injection instead of pre-mix is the oil is engine rpm driven lubrication, not throttle driven. I'll ask if they've seen it with oil injection, 2-cycles. I just don't know.

Bob Wilson

Dana
03-04-2012, 08:06 AM
A related issue I've been wondering about is aerobatics. I'm designing an aerobatic ultralight which will probably use a 447 or similar. I'd hate to have the engine seize coming out of a hammerhead or loop. The aerobatic Quicksilver Super used a 503 (with diaphragm carburetor) and I haven't heard of problems with it, though many 503's had oil injection.

martymayes
03-04-2012, 11:44 AM
Agreed which suggests the failure is not quite as rapid as the Webinar suggested when it discussed transition to cruise leading to prop unloading. Also, I don't know if two-stroke, motorcycles are able to go down large hills and mountains. I have a friend who races them and I'll ask.

This is why I need to ask the webinar speaker a follow-up question. One of the reasons I'm looking at oil injection instead of pre-mix is the oil is engine rpm driven lubrication, not throttle driven. I'll ask if they've seen it with oil injection, 2-cycles. I just don't know.

Bob Wilson

Bob, I have ridden and raced two-stroke dirtbikes and fooled around with small (<50 hp) two-strokes for 40+ yrs. I'd say there is not much correlation between the motorcycle application and airplane application. For starters, a motorcycle has a gearbox where the rider continually changes gear ratios depending on load to keep his engine in the optimal power range. Also, the instantaneous unloading of a bike engine when airborne off a jump is easily controlled by throttle inputs. Airplanes are not flown in the same fashion. I'd be most concerned about putting a load on a two-stroke and not being able to keep the RPM in the proper range. I've trashed a lot of engines that way.

martymayes
03-04-2012, 11:48 AM
A related issue I've been wondering about is aerobatics. I'm designing an aerobatic ultralight which will probably use a 447 or similar. I'd hate to have the engine seize coming out of a hammerhead or loop. The aerobatic Quicksilver Super used a 503 (with diaphragm carburetor) and I haven't heard of problems with it, though many 503's had oil injection.

More anecdotal evidence perhaps but seems like one could examine the engine failures on Rans S-9's and S-10's as they are purposely designed for aerobatics and have reasonable service history available and compare to the straight and level fleet.

I considered the same thing once, installing a two-stroke on something like a Baby Lakes. Outside the relatively low "TBO" I think it would perform satisfactorily but you can almost bet the bank on a power off landing in a relatively high wing load airplane at some point.

Racegunz
03-04-2012, 12:27 PM
In regards to the lean condition while on a descent or during prop unloading manuevers, having flown with a rotax this info is not a secret, everyone who is diligent has indeed moved forward already. The problem arises on a long powered descent, NOT a landing approach on short final at idle power, the idle mixture is rich (or should be) dragging the airplane in on a low final has a possibility of causing a lean condition to occur, and it's a bad practice anyways. No reason to "count" on an engine failure, of course it can happen to any mechanical device, it's too bad that aviation continually has to reinvent the wheel so to speak.

bwilson4web
03-05-2012, 07:24 AM
Thanks!

. . . I'd be most concerned about putting a load on a two-stroke and not being able to keep the RPM in the proper range. . . .
I agree which is why I'm looking at the 3502 which has a relatively flat power-rpm range compared to the nearly identical 3503 that has higher peak power but in a narrower band. A wider power range should allow a larger velocity change with an efficient prop.

What I'll be doing next is to understand the prop-velocity-torque curves for a fixed pitch prop. Then I'll integrate the airframe, engine and prop curves to calculate the expected power:

0-60 mph - acceleration to minimum flying speed at 1,150 lbs
60-90 mph - acceleration to maximum rate of climb speed, at 1,150 lbs
90-140 mph - expected maximum, level flight speed, 1,150 lbs

In regards to the lean condition while on a descent or during prop unloading manuevers, having flown with a rotax this info is not a secret, everyone who is diligent has indeed moved forward already. The problem arises on a long powered descent, NOT a landing approach on short final at idle power, the idle mixture is rich (or should be) dragging the airplane in on a low final has a possibility of causing a lean condition to occur, and it's a bad practice anyways. . . .
Were these failures using carburetor engines?

My plan is to use fuel injection for the 3502 which has an intake, air pressure port to manage the fuel mixture. I'm not sure about an air temperature probe but will check. Regardless, the fuel injection should keep the mixture neither too rich nor too lean and all but eliminate carb ice risk.

I also plan to use engine driven, oil injection. This ties the lubrication to engine RPM regardless of the throttle or fuel consumption. Our four-strokes have an oil pump so it makes sense that a reliable two-stroke would have one too.

Bob Wilson

Racegunz
03-05-2012, 10:00 PM
As far as I know the condition was/is with carbs,not sure how the fuel injection will help since the condition is high rpms with low ammounts of fuel which in a 2 stroke is a coolant just like running rich in a 4 stroke aids cooling. As long as awareness is there and an eye on the EGT's shouldn't be a problem and if this thread is any indication of your deligence I think you'll do fine.

Mike M
03-06-2012, 08:27 AM
I also plan to use engine driven, oil injection. This ties the lubrication to engine RPM regardless of the throttle or fuel consumption. Our four-strokes have an oil pump so it makes sense that a reliable two-stroke would have one too.

why does that make sense? aircraft engines run at pretty constant power most of the time. taxi is the only operation below 65% for most aircraft folks. why include a part which must be purchased, installed, carried around, calibrated, and presents a single-point failure mode when you don't require it? premixing oil&gas eliminates that failure point. granted, it requires thought when refueling. but mechanically it fits with Ford, Stout, Hooten, and Douglas - "simplicate and add lightness"

besides, the hirth 3502 you like doesn't inject oil based on rpm. according to the hirth manual, the oil ratio on that and numerous other models depends on 2 variables:
1. Number of rotations
2. Throttle position (loaded condition)

google.com/url?sa=t&rct=j&q=hirth%203502%20%22oil%20injection%22&source=web&cd=8&ved=0CF4QFjAH&url=http%3A%2F%2Fhirth-motoren.de%2Fde%2Fdownloads-kategorie%2Fcategory%2F25-anleitungen.html%3Fdownload%3D219%253Agetrenntschm ierung&ei=_BJWT7LNHMGrsQLq9tHaCQ&usg=AFQjCNFddxsZ8XUavXr0OfdNOFIQy10X7A&cad=rja

so the oil going into the engine is reduced because the fuel going into the engine is reduced. well, duh, it does that with premix when closing the throttle, too. without added complexity, cost, and weight. plus, if oil injection is better for that engine, why didn't the people who build and warranty the engine require it instead of making it an option? i'd guess they put their money where their engineers are.

your mileage may vary.

bwilson4web
03-06-2012, 10:41 AM
Hi,
. . . why include a part which must be purchased, installed, carried around, calibrated, and presents a single-point failure mode when you don't require it? . . . The oil is there to lubricate the moving parts and needs to be in proportion to the relative velocity of the moving parts to the fixed parts and proportional to the mechanical stress from producing power. The oil pump provides lubrication in proportion to:

rpm - the relative velocity
throttle - the mechanical stress

In contrast, the throttle is not tied directly to the engine rpm and speed of the moving parts. In a descent, the engine and prop will turn rapidly but there won't be a significant flow of pre-mix gas to provide the lubrication for the moving parts. I want my moving engine parts to stay well lubricated regardless of throttle setting and in proportion to the moving part, speed differences.

As for the other concerns, I can afford the oil pump; it will be factory and/or dealer installed; the part and supporting hardware filter, tube, and oil tank appears to be less than 2 lbs; calibration of the oil pump is another one of several calibrations needed for this engine and; the oil pump failure mitigates pre-mix fuel risks and maintenance.

My understanding is pre-mix has about three week expiration date and needs to be drained and replaced. But I'm loath to dispose of the pre-mix by adding to my car tank due the risk to the catalytic converter and O{2} sensors. Furthermore, there are non-trivial safety risks when moving pre-mix around. In contrast, straight gas has a much longer tank storage life. The same is true of the oil in the tank.

I appreciate that fixed operations with a lot of flying and similarly fueled craft can easily handle pre-mix. In contrast, my plane may sit for weeks or longer if health or other demands cut into my flying time. The oil pump avoid the expiration date risk of pre-mix.

Thanks, Bob Wilson

bwilson4web
03-06-2012, 02:03 PM
Propeller pitch analysis revealed:

need for faster prop rpm - to meet the dynamic range of 60-140 mph of the airframe
largest dynamic rpm range 60 hp and above - 60 hp is the rated power of the original engine
It turns out that without doing some machining, I'm limited to the G50 gearbox and a 2.16:1 ratio. Since I'm limited to a 52" prop, this requires a greater pitch which either limits the takeoff-climb or cruise-top speed. . . . There is no free lunch but thankfully, no show stoppers.

Bob Wilson

Frank Giger
03-19-2012, 12:37 AM
Backing up a bit...

"One radical thought is the BSR might fit in front of the firewall. "

I'm not sure putting a chute in the high temp area of the engine compartment is a grand idea.

bwilson4web
03-19-2012, 04:26 AM
Backing up a bit...

"One radical thought is the BSR might fit in front of the firewall. "

I'm not sure putting a chute in the high temp area of the engine compartment is a grand idea.
Good point as the VW installation had a cold-air pipe to a box around the gascolator. Fortunately, the two primary heat sources are well contained, the latest published sketch:
http://hiwaay.net/~bzwilson/dragonfly/2012/firewall_140.jpg
After several interations, this appears to be the most practical approach using the existing engine mount anchor points. The Hirth radiator is to scale.

This version keeps the engine upright and partially covers the stock exhaust and expansion chamber. There are options for a support bracket for the exhaust. There should be no problem with a cold-air feed into a light weight box located high in the steel tube, engine mount frame. This is especially true for the pyrotechnic rocket which needs a similar environment as the gascolator, fuel lines, and fuel injection controller. This would be a more challenging problem if I were using an air cooled engine.

Understand this is a sketch and the final configuration won't be known until after I have all of the firewall-forward equipment: prop, engine, radiator and BSR. Like many things in life, there seems to be some 'margin of error' between the published and actual dimensions and weights.

Having the parts will give me the exact weights and dimensions and allow form-and-fit tacking of the firewall forward assembly. Once the engine mount frame is welded up, we can double check the firewall forward weight and balance. Then we'll use foam to form the cowl plug, fiberglass the cowling, final assembly and check out. That is my plan (and God is laughing.)

Both parachute recovery manufacturers recommend attachments that bring the aircraft 'flat' into the relative motion so the aircraft forms part of the drag needed to slow everything down. The firewall forward installation means these important straps will be shorter and engage early.

Thanks,
Bob Wilson

bwilson4web
03-19-2012, 11:37 PM
Hi,


I'd definitely be interested in seeing such a review Bob. In particular, I'd like to see what kind of risk is incurred by flying at night in a homebuilt airplane with a converted auto powerplant or two-stoke engine.

I finally got a little time today to put fingers to keyboard so I've posted the mission requirements review over in "Homebuilders Corner." Whether or not it does any good, well that is how the process works.

Enjoy!

Bob Wilson